1. Field of the Invention
This invention relates to a static convertor means of the kind comprising a d.c. voltage capacitor, a static convertor including at least one phase group with forced commutation connected between two busbars and two commutating inductors, common to the at least one phase group, connected between the d.c. voltage capacitor and respective ones of the busbars, each phase group having two series-connected branch portions, the point of connection therebetween constituting a phase terminal of the group, each branch portion comprising a semiconductor means which is controllable in one direction and conducting in the other direction, and a two-terminal network with an impedance which is inductive at higher frequencies, said network being connected to the convertor for transporting energy between the two-terminal network and the convertor. Hereinafter such a convertor means will be referred to as a "convertor means of the kind referred to".
2. Description of the Prior Art
A previously known convertor means of the kind referred to is shown in FIG. 1. In this known convertor means the convertor (SR1) is a three-phase self-commutated inverter and has two busbars (P, N) between which the three phase groups are connected. Each phase group consists of a series connection of two bridge branch portions (e.g. TR1, DR1 and TR2, DR2, respectively), each branch portion consisting of a thyristor and a diode connected in anti-parallel with the thyristor. The phase group for phase R thus consists of the thyristors TR1, TR2 and the diodes DR1, DR2. The connection point of the bridge branch portions constitutes phase terminal R of the inverter. The phase groups for phases S and T are identical with the phase group for phase R. The inverter has a common commutating circuit which comprises commutating capacitors CK1, CK2 and commutating inductors LK1, LK2, the two capacitors CK1, CK2 being connected in series with each other between d.c. voltage terminals PD and ND of the bridge. Each one of the commutating inductors LK1, LK2 is connected between a respective one of the busbars P and N and a respective one of the d.c. voltage terminals PD and ND of the inverter. Each phase group has two antiparallel-connected commutating thyristors, which are connected between the connection point of the commutating capacitors and the phase terminal of the phase group. The commutating thyristors for the phase group R are designated KR1 and KR2, and the commutating thyristors for the other two phase groups are designated in a corresponding way. The commutating thyristors are controlled in a manner known per se so that recharge of the commutating capacitors and extinction of the thyristors of each phase group can be achieved at the desired times during each period. The inverter is controlled in such a manner, known per se, that each phase group delivers a rectangular voltage on its phase terminal with a duration of 180.degree.. The three phase groups are controlled with a mutual phase displacement of 120.degree.. In a manner known per se, the voltage of each phase group can possibly be controlled to the desired amplitude and/or to approximate more closely to a sinusoidal form by being pulse-width modulated with the aid of additional commutations within each half-period.
In the known convertor means of the kind referred to and shown in FIG. 1, a d.c. voltage capacitor CD is connected between the d.c. voltage connections PD and ND of the inverter and constitutes a smoothing capacitor for the intermediate link d.c. voltage.
A two-terminal network TP with inductive impedance is formed by a diode rectifier bridge DB which has a smoothing inductor LD on its d.c. side. The a.c. voltage connections of the diode bridge are intended for connection to a three-phase a.c. voltage network (not shown). The connection points A and B of the two-terminal network are connected to the d.c. voltage connections PD and ND of the inverter. Alternatively, of course, the diode bridge DB may be a single-phase bridge or be replaced by a single-phase or multi-phase thyristor convertor or other d.c. voltage source.
In known convertor means of the kind referred to and as shown in FIG. 1, a major proportion of the losses occur in the commutating inductors LK1 and LK2.
FIG. 1A shows representative curve shapes for the convertor means shown in FIG. 1 during no load operation. The curves designated R, S and T show the control orders to the control pulse device during one period. The remaining curves show the currents and/or voltages appearing at various other parts of the circuit of the convertor means. Upon an order for changing a phase (e.g. R), a reversal of the polarity of the capacitor voltage is initiated by a turn-off thyristor (KR1, KR2) being supplied with a control pulse. A current arises through the diode, which is connected in antiparallel with the main thyristor which is to be extinguished, whereby this thyristor recovers, that is, resumes its off-state voltage capacity. The reversal is completed by the other main thyristor in the same phase being given a firing pulse, whereby a second current pulse occurs, so that the capacitor is made ready to carry out the next commutation.